1. Field of the Invention
This invention relates to the field of hard disk drive suspensions. More particularly, this invention relates to improved baseplate hub geometry to achieve reduced after-swaging deflection of the suspensions while maintaining sufficient retention torque.
2. Description of Related Art
An actuator is a mechanism for moving disk drive suspensions linearly into position at a disk surface. The actuator E-block has one or more arms to which the head gimbal assembly load beams are attached, usually two load beams per arm, one above, the upper load beam, and one below, the lower load beam. The load beams are fixed by welding to the planar base of a mounting plate that has an annular boss rising from the base. The lower load beam is attached to a lower mounting plate base and the lower mounting plate base is inserted into the lower opening of a bore that extends through the arm from its lower face to its upper face. The upper and lower mounting plates thus are reversed in orientation and have their respective bosses opposed and differently affected by the staking (swaging) operation. The terms upper and lower herein as applied to mounting plates refer to the direction of swaging. Thus the mounting plate that is closer to the origin of the swaging movement is the upper mounting plate and the mounting plate that is closer to the destination of the swaging movement is the lower mounting plate.
To secure the oppositely directed mounting plates to the actuator arm, the mounting plate bosses are radially expanded to engage the bore interior wall by swaging. Typically, the swaging step includes forcing two swage balls through the mounting plate bosses to expand them. The first and smaller swage ball expands the bosses to a first extent, and the second and larger swage ball expands the bosses even more until the bosses are firmly pressed up against the interior wall of the actuator arm bore.
There is in addition to the radial force being exerted a downward axial force associated with the movement of the staking tool. This force has a different effect on the upward facing (lower) mounting plate, which tends to deform convex toward the bore, from its effect on the downward facing (upper) mounting plate, which tends to deform concave toward the bore.
Unfortunately, the swaging process can result in distortion of the mount plate and actuator arm, and hence misalignment of the suspension from the desired angle. The different deformation effects on the upper and lower mounting plates cause different and difficult to control changes in the shape of the mounting plates and their intended suspension gram preloads. Miniaturization and tighter tolerance requirements have made distortion after the swaging operation increasingly problematic. The distortion produces deviation from the target suspension gramload value, which is a critical parameter for the performance and functionality of a suspension assembly. In addition to gramload changes, two other important aspects of a successful swaging operation are retention torque and swage defect. Retention torque is the amount of torque applied to the suspension that overcomes the staking of the mount plate to the actuator arm. The retention torque must therefore be sufficiently high to prevent suspension failure. As overall dimensions get smaller, providing a baseplate design that will exhibit a sufficiently high retention torque has become a challenging task.
Several patents have attempted to address the after-swaging deflection problem. U.S. Patent Publication No. 2006/0174472 by Zhang et al. purports to provide a method of reducing the effect of arm distortion after swaging, by providing a baseplate hub design such that the contact after swaging is close to the neutral axis of the arm. The methodology appears to apply to an actuator arm with one suspension attached. U.S. Pat. No. 7,190,555 issued to Hanrahan et al. suggests that a thicker arm compared to the baseplate hub outer diameter will provide more rigidity and thus less distortion after swaging. U.S. Pat. No. 6,183,841 issued to Hanrahan et al. purports to provide a general criterion for a low profile baseplate design with good retention torque after swaging. U.S. Pat. No. 6,297,934 issued to Coon purports to reduce after-swaging gram load differentials by providing a sloped actuator arm, thereby providing a different amount of mechanical support to the upper suspension than to the lower suspension. U.S. Pat. No. 7,130,156 issued to Fossum purports to increase retention torque by providing a swaging hole with a tapered portion that increases in diameter in a direction away from the flange portion.